1. Field of the Invention
The present invention relates to a printed-wiring substrate having connection terminals, and more particularly, to a printed-wiring substrate having connection terminals on each of main and back faces thereof.
2. Description of the Related Art
Conventionally, a printed-wiring substrate having connection terminals on each of main and back faces thereof has been known. FIG. 11 shows an example of a printed-wiring substrate 101.
The printed-wiring substrate 101 generally has a plate-like shape, and has a main face 101A and a back face 101B. A large number of main-face-side connection terminals 103 are formed on the main face 101A; and a large number of back-face-side connection terminals 105 are formed on the back face 101B. An IC chip 111 indicated by a broken line in FIG. 11 can be mounted on the main face 101A by solder-bonding connection terminals 113 of the IC chip 111 to the main-face-side connection terminals 103. Similarly, a motherboard 121, indicated by a broken line in FIG. 11, can be connected to the back face 101B by solder-bonding connection terminals 123 of the motherboard 121 to the back-face-side connection terminals 105.
To prevent oxidation, each of the main-face-side connection terminals 103 of the printed-wiring substrate 101, which are formed of copper connection pads, is covered with an Ni plating layer having a thickness of about 3 to 7 xcexcm, and is further covered with a thin Au plating layer having a thickness of about 0.05 xcexcm.
The reason for thinning the Au plating layer is that if the Au plating layer is thick, an inter-metallic compound is produced when the main-face-side connection terminals 103 and the connection terminals 113 of the IC chip 111 are soldered together, resulting in deteriorated solderability and connection reliability. Specifically, during the solder-bonding process, Au-Sn, which is a hard and brittle inter-metallic compound, is produced at the interface between solder and the main-face-side connection terminals 103 due to diffusion of Au contained in the Au plating layer of the main-face-side connection terminals 103 and Sn contained in solder. This reduces the bonding strength between the printed-wiring substrate 101 and the IC chip 111. Accordingly, the Au plating layer is preferably rendered as thin as possible in order to solve the problem caused by production of an inter-metallic compound, to thereby enhance connection reliability with the main-face-side connection terminals 103.
Similarly, each of the back-face-side connection terminals 105 of the printed-wiring substrate 101, which are formed of copper connection pads, is covered with an Ni plating layer having a thickness of about 3 to 7 xcexcm, and is further covered with a thin Au plating layer having a thickness of about 0.05 xcexcm.
A printed-wiring substrate 201 as shown in FIG. 12 has recently been developed as an improvement of the above-described printed-wiring substrate 101.
In the printed-wiring substrate 201, main-face-side connection terminals 203 are formed on the main face 201A; and back-face-side connection terminals 205 are formed on the back face 201B. An IC chip 211 can be mounted on the main face 201A by solder-bonding connection terminals 213 of the IC chip 211 to the main-face-side connection terminals 203. Similarly, a motherboard 221 can be connected to the back face 201B not by means of solder, but through use of a socket or the like which establishes mechanical contact between connection terminals 223 of the motherboard 221 and the back-face-side connection terminals 205.
3. Problems to be Solved by the Invention
When the printed-wiring substrate 201 shown in FIG. 12 is used, the back-face-side connection terminals 205 are forcedly brought into contact with the connection terminals 223 of the motherboard 221, and the contact may cause exfoliation of the Au plating layer formed on the surfaces of the back-face-side connection terminals 205. In such a case, the Ni plating layer is exposed from portions at which the Au plating layer has exfoliated. This may result in problems such as an increase in the contact resistance between the back-face-side connection terminals 205 and the connection terminals 223 of the motherboard 221.
The present invention has been achieved in view of the foregoing. It is therefore an object of the invention to provide a printed-wiring substrate capable of increasing connection reliability with an electronic component to be mounted on the main face as well as the connection reliability with another substrate to be connected to the back face of the printed-wiring substrate. Another object of the present invention is to provide a method for fabricating the printed-wiring substrate.
The above first object of the present invention has been achieved by providing a printed-wiring substrate having a substantially plate-like shape, a main face and a back face, and comprising main-face-side connection terminals for solder-bonding to connection terminals of an electronic component which is to be mounted on the main face; and back-face-side connection terminals for connecting, through mechanical contact, to connection terminals of another substrate which is to be connected to the printed-wiring substrate on the back-face side thereof, wherein the surfaces of the main-face-side connection terminals are coated with a main-face-side Au plating layer; and the surfaces of the back-face-side connection terminals are coated with a back-face-side Au plating layer which is thicker than the main-face-side Au plating layer.
In the printed-wiring substrate of the present embodiment having main-face-side connection terminals for solder-bonding to connection terminals of an electronic component and back-face-side connection terminals for connecting, through mechanical contact, to connection terminals of another substrate, the main-face-side Au plating layer is rendered relatively thin, and the back-face-side Au plating layer is rendered relatively thick.
Accordingly, only a small amount of inter-metallic compound is produced from Au of the main-face-side connection terminals and Sn contained in solder when the main-face-side connection terminals and the connection terminals of the electronic component are soldered together. Therefore, the solderability of the main-face-side connection terminals is enhanced, so that the reliability of connection between the main-face-side connection terminals and the connection terminals of the electronic component is improved. In addition, since the back-face-side Au plating layer has an increased thickness, the problem of exfoliation of the back-face-side Au plating layer with resultant exposure of a base layer and increased contact resistance does not occur even when the back-face-side connection terminals are connected to the connection terminals of another substrate through mechanical contact. Therefore, the connection reliability between the back-face-side connection terminals and the connection terminals of another substrate can be improved as well.
Moreover, in the above-described printed-wiring substrate, the main-face-side Au plating layer preferably has a thickness of not less than 0.03 xcexcm but not greater than 0.12 xcexcm; and the back-face-side Au plating layer preferably has a thickness of not less than 0.2 xcexcm.
In the present embodiment, since the main-face-side Au plating layer preferably has a thickness of not less than 0.03 xcexcm but not greater than 0.12 xcexcm, oxidation of the main-face-side connection terminals can be reliably prevented. In addition, the amount of inter-metallic compound (e.g., Auxe2x80x94Sn inter-metallic compound) produced at the time of soldering the main-face-side connection terminals and the connection terminal of the electronic component can be reduced. Therefore, the solderability between the main-face-side connection terminals and the connection terminals of the electronic component can be further enhanced, and the reliability of the connection can be further improved.
Meanwhile, since the back-face-side Au plating layer has a thickness of not less than 0.2 xcexcm, exfoliation of the back-face-side Au plating layer is prevented due to mechanical contact between the back-face-side connection terminals and the connection terminals of another substrate. Therefore, the solderability between the back-face-side connection terminals and the connection terminals of another substrate can be further enhanced, and the reliability of the connection can be further improved.
The above second object of the present invention has been achieved by providing a method for fabricating a printed-wiring substrate having a substantially plate-like shape, a main face and a back face, and comprising main-face-side connection terminals for solder-bonding to connection terminals of an electronic component which is to be mounted on the main face; and back-face-side connection terminals for connecting, through mechanical contact, to connection terminals of another substrate which is to be connected to the printed-wiring substrate on the back-face side thereof. The method comprises a first Au plating step of forming a first main-face-side Au plating layer on the surfaces of the main-face-side connection terminals and a first back-face-side Au plating layer on the surfaces of the back-face-side connection terminals; a masking step of covering the first main-face-side Au plating layer with a protection layer; a second Au plating step of forming a second back-face-side Au plating layer on the first back-face-side Au plating layer; and a protection layer removal step of removing the protection layer after completing the second Au plating step.
According to the present embodiment, after forming a first main-face-side Au plating layer and a first back-face-side Au plating layer in the first Au plating step, the first main-face-side Au plating layer is covered with a protection layer, and in the second Au plating step, a second back-face-side Au plating layer is formed on the first back-face-side Au plating layer only.
Therefore, in a printed-wiring substrate having main-face-side connection terminals for solder-bonding to connection terminals of an electronic component and back-face-side connection terminals for connecting, through mechanical contact, to connection terminals of another substrate, the Au plating layer of the main-face-side connection terminals (the first main-face-side Au plating layer) can be formed to have a relatively small thickness; and the Au plating layer of the back-face-side connection terminals (the first back-face-side Au plating layer and the second back-face-side Au plating layer) can be formed to have a relatively large thickness.
Such a printed-wiring substrate can enhance the solderability between the main-face-side connection terminals and the connection terminals of the electronic component, so that the connection reliability can be improved. In addition, the reliability of the connection between the back-face-side connection terminals and the connection terminals of another substrate can be improved.
Further, in the present embodiment, since the second back-face-side Au plating layer is formed on the first back-face-side Au plating layer immediately after the masking step, without removing the first back-face-side Au plating layer, productivity of the printed-wiring substrate can be improved.
Moreover, in the above-described method of fabricating a printed-wiring substrate, the first Au plating is preferably first displacement Au plating; and the second Au plating is preferably second displacement Au plating.
In many cases, the main-face-side connection terminals for connection with an electronic component and the back-face-side connection terminals for connection with another substrate are both formed in such a manner that individual terminals are electrically isolated from one another. Therefore, electroless Au plating is preferably used as the first Au plating and the second Au plating.
However, since reduction Au plating, which is one type of electroless Au plating, is frequently performed using a strong-alkaline plating solution containing potassium cyanoaurate, during plating, components of the protection layer may be dissolved into the plating solution, resulting in variation in plating conditions. Further, in the case in which a printed-wiring substrate formed of resin is fabricated, a solder resist layer may be damaged by the plating solution, or the plating solution may penetrate under the solder resist layer, resulting in generation of so-called harrowing.
By contrast, in the present embodiment, the first displacement Au plating and the second displacement Au plating are performed using an acidic or neutral plating solution, and thus the problem of dissolution of the protection layer with resultant variation in plating conditions and the problem of harrowing can be avoided.
The above second object of the present invention has also been achieved by providing a method for fabricating a printed-wiring substrate having a substantially plate-like shape, a main face and a back face, and comprising main-face-side connection terminals for solder-bonding to connection terminals of an electronic component which is to be mounted on the main face; and back-face-side connection terminals for connecting, through mechanical contact, to connection terminals of another substrate which is to be connected to the printed-wiring substrate on the back-face side thereof. The method comprises a first Au plating step of forming a first main-face-side Au plating layer on the surfaces of the main-face-side connection terminals and a first back-face-side Au plating layer on the surfaces of the back-face-side connection terminals; a masking step of covering the first main-face-side Au plating layer with a protection layer; a first back-face-side Au plating layer removal step of removing the first back-face-side Au plating layer after completing the masking step; a second Au plating step of forming a second back-face-side Au plating layer on the surfaces of the back-face-side connection terminals, from which the first back-face-side Au plating layer has been removed, to a thickness greater than that of the first main-face-side Au plating layer; and a protection layer removal step of removing the protection layer after completing the second Au plating step.
According to the present embodiment, after forming a first main-face-side Au plating layer and a first back-face-side Au plating layer in the first Au plating step, the first main-face-side Au plating layer is covered with a protection layer, and only the first back-face-side Au plating layer is removed. Subsequently, in the second Au plating step, a second back-face-side Au plating layer is formed to a thickness greater than that of the first main-face-side Au plating layer.
Therefore, in a printed-wiring substrate having main-face-side connection terminals for solder-bonding to connection terminals of an electronic component and back-face-side connection terminals for connecting, through mechanical contact, to connection terminals of another substrate, the Au plating layer of the main-face-side connection terminals (the first main-face-side Au plating layer) can be formed to have a relatively small thickness; and the Au plating layer of the back-face-side connection terminals (the second back-face-side Au plating layer) can be formed to have a relatively large thickness.
Such a printed-wiring substrate can enhance the solderability between the main-face-side connection terminals and the connection terminals of the electronic component, so that the connection reliability can be improved. In addition, the reliability of the connection between the back-face-side connection terminals and the connection terminals of another substrate can be improved.
Further, because the first back-face-side Au plating layer is removed before forming the thick second back-face-side Au plating layer, this prevents fabrication conditions from becoming unstable (e.g., where formation of an Au plating layer on the first back-face-side Au plating layer becomes difficult), which unstable conditions would otherwise result when the first back-face-side Au plating layer is exposed to atmospheric air or high temperature during the masking step. Therefore, the thick second back-face-side Au plating layer can be reliably formed in the second Au plating step.
Moreover, in the above-described method of fabricating a printed-wiring substrate, the first Au plating is preferably first displacement Au plating; and the second Au plating is preferably second displacement Au plating.
In the present embodiment, the first displacement Au plating and the second displacement Au plating are performed using an acidic or neutral plating solution, and thus the problem of dissolution of the protection layer with resultant variation in plating conditions is prevented. Further, in the case in which a printed-wiring substrate formed of resin is fabricated, the problems of solder resist layer damage by the plating solution and harrowing can be avoided.
Moreover, in the above-described method of fabricating a printed-wiring substrate, the second Au plating step preferably comprises a second thin displacement Au plating step of forming a second back-face-side thin Au plating layer on the surfaces of the back-face-side connection terminals, from which the first back-face-side Au plating layer has been removed; and a second thick displacement Au plating step of forming a second back-face-side thick Au plating layer on the second back-face-side thin Au plating layer.
In the present embodiment, a second back-face-side thin displacement Au plating layer is formed on the back-face-side connection terminals, from which the first back-face-side Au plating layer has been removed, and then a second back-face-side thick displacement Au plating layer is formed on the second back-face-side thin displacement Au plating layer.
When the Au plating layer is formed in two steps as described above, variation in thickness of the back-face-side Au plating layer (the second back-face-side thin displacement Au plating layer and the second back-face-side thick displacement Au plating layer) can be suppressed. Further, formation of the second back-face-side thin displacement Au plating layer shortens the plating time necessary for forming the second back-face-side thick displacement Au plating layer.